Improve benchmark with statistical analysis and fair comparison

Changes:
- Move benchmark files to dedicated /benchmark directory
- Add run_benchmark.sh script for easy execution with uvx
- Improve benchmark.py with:
  - Statistical analysis: 10 runs with mean, median, std dev, min, max
  - Fair comparison: Both libraries test equivalent configuration complexity
  - System information: Display Python version, platform, library versions
  - Better warmup: 50 iterations before timing
  - Raw data output: All timing data for verification
  - Comprehensive documentation

Benchmark improvements:
- Test realistic configuration with 18 fields across multiple categories
- msgspec-ext: Flat structure with __ delimiter (optimized for .env)
- pydantic-settings: Nested structure with BaseSettings classes
- Both load from identical .env file

Updated README with accurate benchmark results:
- 2.7x faster than pydantic-settings (down from 3.8x claim)
- Based on 10 runs × 1000 iterations with statistical validation
- More honest and reproducible results

Run: ./benchmark/run_benchmark.sh to reproduce results

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vilsonrodrigues

README.md

@@ -142,26 +142,26 @@ class AppSettings(BaseSettings):
 
 msgspec-ext leverages msgspec's high-performance serialization with bulk JSON decoding for maximum speed.
 
-**Benchmark Results** (1000 iterations, Python 3.12):
+**Benchmark Results** (10 runs × 1000 iterations, Python 3.12):
 
 | Library | Time per load | Relative Performance |
 |---------|---------------|---------------------|
-| msgspec-ext | 0.702ms | Baseline ⚡ |
-| pydantic-settings | 2.694ms | 3.8x slower |
+| msgspec-ext | 2.271ms | Baseline ⚡ |
+| pydantic-settings | 6.157ms | 2.7x slower |
 
-msgspec-ext is **3.8x faster** than pydantic-settings while providing the same level of type safety and validation.
+msgspec-ext is **2.7x faster** than pydantic-settings while providing the same level of type safety and validation.
 
 **Key optimizations:**
 - Bulk JSON decoding in C (via msgspec)
 - Cached encoders and decoders
 - Automatic field ordering
 - Zero Python loops for validation
 
-*Benchmark measures complete settings initialization including .env file parsing and type validation. Run `python benchmark.py` to reproduce.*
+*Benchmark measures complete settings initialization with complex configuration (app settings, database, redis, feature flags) including .env file parsing and type validation. Run `./benchmark/run_benchmark.sh` to reproduce.*
 
 ## Why msgspec-ext?
 
-- **Performance** - 3.8x faster than pydantic-settings
+- **Performance** - 2.7x faster than pydantic-settings
 - **Lightweight** - 4x smaller package size (0.49 MB vs 1.95 MB)
 - **Type safety** - Full type validation with modern Python type checkers
 - **Minimal dependencies** - Only msgspec and python-dotenv
@@ -172,7 +172,7 @@ msgspec-ext is **3.8x faster** than pydantic-settings while providing the same l
 |---------|------------|-------------------|
 | .env support | ✅ | ✅ |
 | Type validation | ✅ | ✅ |
-| Performance | **3.8x faster** ⚡ | Baseline |
+| Performance | **2.7x faster** ⚡ | Baseline |
 | Package size | 0.49 MB | 1.95 MB |
 | Nested config | ✅ | ✅ |
 | Field aliases | ✅ | ✅ |